Duplex sealing process

ABSTRACT

An anodized aluminum coating is sealed by a duplex process that comprises immersing the coating in a solution containing a divalent metal cation such as iron, cobalt or nickel followed by immersing the coating in a solution containing a multivalent anion such as phosphate, silicate, molybdate or chromate. The resulting seal imparts greatly improved corrosion resistance to the coating.

United States Patent 1191 Fassell Dec. 3, 1974 52 US. or 148/6.l5, 148/616, 148/6.l4, 204/35 N, 204/38 A 51 Int. Cl. C236 1/10, c231 7/08, C23b 5/50 [58] Field of Search 148/6.l5 R; 6.15 z, 6.16; 204/35 N, 38 A [56] References Cited UNITED STATES PATENTS 3,099,610 7/1963 Cybriwsky et al. 204/35 N Asada et al. 204/38 A Gurev et al. l48/6.i5 R

Primary ExaminerJohn D. Welsh Attorney, Agent, or Firm-Robert D. Sanborn; William F. Johnson 5 7] ABSTRACT An anodized aluminum coating is sealed by a duplex process that comprises immersing the coating in a solution containing a divalent metal cation such as iron, cobalt or nickel followed by immersing the coating in a solution containing a multivalent anion such as phosphate, silicate, molybdate or chromate. The resulting seal imparts greatly improved corrosion resistance to the coating.

10 Claims, No Drawings DUPLEX SEALING PROCESS BACKGROUND OF THE INVENTION Anodized coatings are produced on aluminum and cation, such as the sodium or ammonium salt of the anion. During immersion of the anodized coating in the second solution, the temperature of the second solution preferably is maintained within a temperature range of aluminum alloys for functional purposes such as in- 5 about 140F 212F and a pH range of about 2 l0. creased corrosion resistance, electrical insulation, Tests indicate that optimum corrosion resistance is abrasion resistance, chemical inertness and adsorption, achieved when the secondsolutiorTisiiiairitairfiad at a or for purely decorative purposes since the anodized higher temperature than the first solution. coating generally is visually aesthetic. Improved aes- Careful rinsing is conducted between immersion in thetics can be achieved readily since the anodized coatthe first solution and the second solution of the process ing adsorbs any of a wide variety of dyes or other colorof this invention to avoid contamination of the second ing agents. Excellent adhesion also can be obtained besolution with any of the divalent metal cations from the tween ordinary paints and the anodized coatings. first solution. Other supporting operations typically An anodized aluminum coating typically is extremely used in sealing anodized coatings also can be conporous and various techniques have been developed for [5 ducted. The process apparently is effective on anodized sealing the anodized coating to prevent corrosive atcoatings produced by any known technique; anodized tack and improve coating durability. The usual sealing Coatings P ced in ypical ac d anodiz ng baths such process involves a one-step immersion of the coating in as sulfuric acid, phosphoic acid or oxalic acid exhibit boiling water or a solution of potassium dichromate, no significant variations in the effectiveness of the prosodium molybdate, sodium dihydrogen phosphate or cess. nickel acetate. Such one-step operations do in fact pro- One important advantage of the process is its ability duce significant sealing of the anodized coating and reto produce a colorless seal having extremely good corsult in improved corrosion resistance. rosion resistance. Such colorless seals can be produced with any of the aforementioned solutions except those SUMMARY OF THE INVENTION in which the second solution contains chromate or di- This invention provides a surprising improvement on chromate multivalent anions. Minor shading can be the one-step sealing processes of the prior art by a duachieved if desired. with certain of the other solutions; plex or two-step proc s that c mprises fir t imme i for example the use of certain concentrations of ferrous the anodized coating in a solution of a salt that dissociammonium sulfate in the first solution can produce a ates in the solution to produce a divalent metal cation, tan shape. and subsequently immersing the coating into a cond Analysis indicates that the mechanism of the duplex solution of a salt that dissociates to produce a multivaling pro ss Of this invention is based on the apparlent anion. The first solution is free of multivalent anint availability o two types of adsorption sites in the ons containing atoms of oxygen and atoms of molybdeanodized coating. One of the sites appears to be a divanum phosphorous, silicon, chlorine or chromium. One lent cation acceptor type whereas the second is a multiof these multivalent anions must be present in the secvalent anion acceptor type. The duplex process of the 0nd solution which can contain monovalent cations but invention satisfies both of the Sites and reduces the mopreferably is free of any significant amount of divalent lecular spacing in the anodized coating to the point t l ti n where the diffusion rate of corrosion causing agents is An aqueous solvent typically is used for each of the diminished considerably. Reversing the sequence of the solutions. Typical divalent metal cations for the first 50- steps of the PrOCeSS pr es nfe o Corrosion resislution include divalent cations of iron, cobalt, nickel, tahee as dOeS Combining e di a e t etal Cations with lead, magnesium, zinc, cadmium, barium, cal i the multivalent anions in the same solution. The reason opper, manga str ti m a d tin A di o i ti for this is uncertain although it is theorized that the organic salt of the divalent metal such as its acetate or mu nt arliOnS tend to block at least temporarily propionate or a dissociative inorganic salt such as its the cation acceptor sites, and thus leave spaces suscepsulfate, chloride, bromide, iodide, or nitrate can be tible to corrosion causing agents. used to produce the first solution. During immersion of the anodized coating in the first solution, the first solu- DETAiLED CRIPTION yp y maintaained i i a temperature range The solutions for carrying out the duplex process of of about 120 200 F and )Vtthm 2 P f of about this invention are prepared in a conventional manner 2 Best results are achieved Wlth a divalent metal dissolving appropriate salts in water concat on of iron, cobalt or nickel. Tests indicate t centrations range from about 1 to 7 grams of salt per mamtammg the first Solutlon a temperature liter. The solutions are then heated to the operating range of about 160F 180F during the immersion op- ,temperatures tlmizes t resulting correston resistance Panels of various aluminum alloys including nos. A typical multivalent anion useful in the second solu- 2024, 6061 d 7075 were treated i h fi t and tiO include molybdat aci ph phat phosphate, ond solutions listed in Table I. None of the anodized chromate and diehfomate, Silicate and Chlorate The coatings failed from corrosion when exposed for 240 Second Solution preferably is made from a l that hours to a 5 percent salt fog in accordance with Federal sociates into the multivalent anion and a monovalent T M h d Standard 141 TABLE I i A First Solution Second Solution Salt and conc. Temp. and time Salt and cone. Temp. and time Ni(c,H,o,i,. l.25g/l m min, Nii,Muo,.2H,0. M r/i mini 2 min. Co(C,H;O,) l.25g/l I6()F l() min. Nn,MoO,.2H,O. 6.5g I60F 2 min. FeSOJNHU S04. I ZSgII 160F l() min. Na,MoO .2H,O. 6.5g" |60F 2 min.

B E pea-mafia First Solution Second Solution Salt and cone. Temp. and time Salt and cone. Temp. and time Ph((-,H;,O 2.5g/l loF l0 min. Na MoO 2H O, 6.5g/l 160F 2 rninv Mg(C H;,O llfigll l6()F min. Nu,Muo,.2H,o. 6.5g/l 160F 2 min. Zn(CgH Og)2 l.2Sg/l l60F ll) min. Na MoO .2H O, 6.5g/l 160F 2 min. Cd(C l-l -.O l.25g/l l60F 10 min. Nu M0O .2H O, 6.5g/l l60F 2 min. Ni(C H O,J.,, 125g]! l60F l0 min. K H,PO,, 4.12g/l l60F 2 min. Ni(( H,-,O,) LZSg/l l60F l0 min. K PO 4.12gil l60F 2 min. NHCQH OQ 1.25g/l l60F 10 min. Na Cr O 4.12g/l l60F 2 min.

For comparative purposes, panels treated according anion selected from molybdateI acid phosphate,

to the duplex process of this invention were tested along with panels treated with either one of the solutions and with panels sealed with pure deionized water. The panels were exposed to N 0 in the presence of water vapor (approximately 2 ml of N 0 and 2 ml of water vapor at 70F) for one hour, and then were subjected to 240 hours of exposure to 5 percent salt fog in accordance with Federal Test Method 141. Table 2 lists the test results. Note that test number 4 and 5, which were treated with the duplex process of this invention, exhibited a failure frequency well below the frequency obtained with the other tests.

Thus the duplex sealing process of this invention provides greatly improved corrosion resistance for anodized aluminum coatings. The duplex process is capable of producing colorless seal coatings and is particularly useful on anodized articles intended for the heavy service encountered in automotive applications.

I claim:

1. A process for improving the corrosion resistance of an anodized aluminum coating of an aluminum based article comprising:

a. contacting said anodized coating with a first aqueous solution of salt selected from the group consisting of acetates, propionates, sulphates, chlorides, bromides, iodides and nitrates, which salt disassociates in water to form a divalent cation selected from the group consisting of iron, cobalt, nickel, lead, magnesium, zinc, cadmium, barium, calcium, copper, manganese, strontium and tin;

b. rinsing said anodized coating; and

c. contacting said anodized coating with a second aqueous solution of a sodium, potassium or ammonium salt which disassociates in water to form an phosphate, chromate, dichromate, silicate and chlorate.

2. The process of claim 1 wherein said first aqueous solution is maintained within a pH range of about 2 to about 6.5 during contact with said anodized coating and said second aqueous solution is maintained within a pH range of about 2 to about 10 during contact with said anodized coating.

3. The process of claim 1 wherein said second aqueous solution is maintained at a higher temperature than said first aqueous solution during contact with said an- .odized coating.

4. The process of claim 1 wherein said anodized coating is-contacted with said first and second aqueous solutions by immersing said aluminum based article in said solutions.

5. The process of claim 4 wherein said first aqeuous solution is maintained within a temperature range of about F to about 200F during immersion therein of said aluminum based article, and said second solution is maintained within a temperature rangeof about F to about 212F during immersion therein of said aluminum based article.

6. The process of claim 5 wherein said first aqueous solution is maintained within a pl-l range of about 2 to about 6.5 during immersion therein of said aluminum based article and said second aqueous solution is maintained within a pH range of about 2 to about 10 during immersion therein of said aluminum based article.

7. The process of claim 6 wherein the cation of said first solution is a divalent cation of iron, cobalt or nickel and the anion of the second aqueous solution is selected from the group consisting of molybdate, acid phosphate, silicate, chromate and dichromate.

8. The process of claim 7 wherein said second aqueous solution is maintained at a higher temperature than said first aqueous solution during immersion of said aluminum based article in the respective solutions.

9. The process of claim 1 wherein the cation of said first aqueous solution is a divalent cation of iron, cobalt or nickel.

10. The process of claim 1 wherein the anion of said second aqueous solution is selected from the group consisting of molybdate, acid phosphate, silicate, chromate and dichromate. 

1. A PROCESS FOR IMPROVING THE CORROSION RESISTANCE OF AN ANODIZED ALUMINUM COATING OF AN ALUMINUM BASED ARTICLE COMPRISING: A. CONTACTING SAID ANODIZED COATING WITH A FIRST AQUEOUS SOLUTION OF SALT SELECTED FROM THE GROUP CONSISTING OF ACETATES, PROPIONATES, SULPHATES, CHLORIDES, BROMIDES, IODIDES AND NITRATES, WHICH SALT DISASSOCIATES IN WATER TO FORM A DIVALENT CATION SELECTED FROM THE GROUP CONSISTING OF IRON, COBALT, NICKEL, LEAD, MAGNESIUM, ZINC, CADMIUM, BARIUM, CALCIUM, COPPER, MANGANESE, STRONTIUM AND TIN; B. RINSING SAID ANODIZED COATING; AND C. CONTACTING SAID ANODIZED COATING WITH A SECOND AQUEOUS SOLUTION OF A SODIUM, POTASSIUM OR AMMONIUM SALT WHICH DISASSOCIATES IN WATER TO FORM AN ANION SELECTED FROM MOLYBDATE, ACID PHOSPHATE, CHROMATE, DICHROMATE, SILICATE AND CHLORATE.
 2. The process of claim 1 wherein said first aqueous solution is maintained within a pH range of about 2 to about 6.5 during contact with said anodized coating and said second aqueous solution is maintained within a pH range of about 2 to about 10 during contact with said anodized coating.
 3. The process of claim 1 wherein said second aqueous solution is maintained at a higher temperature than said first aqueous solution during contact with said anodized coating.
 4. The process of claim 1 wherein said anodized coating iS contacted with said first and second aqueous solutions by immersing said aluminum based article in said solutions.
 5. The process of claim 4 wherein said first aqeuous solution is maintained within a temperature range of about 120*F to about 200*F during immersion therein of said aluminum based article, and said second solution is maintained within a temperature range of about 140*F to about 212*F during immersion therein of said aluminum based article.
 6. The process of claim 5 wherein said first aqueous solution is maintained within a pH range of about 2 to about 6.5 during immersion therein of said aluminum based article and said second aqueous solution is maintained within a pH range of about 2 to about 10 during immersion therein of said aluminum based article.
 7. The process of claim 6 wherein the cation of said first solution is a divalent cation of iron, cobalt or nickel and the anion of the second aqueous solution is selected from the group consisting of molybdate, acid phosphate, silicate, chromate and dichromate.
 8. The process of claim 7 wherein said second aqueous solution is maintained at a higher temperature than said first aqueous solution during immersion of said aluminum based article in the respective solutions.
 9. The process of claim 1 wherein the cation of said first aqueous solution is a divalent cation of iron, cobalt or nickel.
 10. The process of claim 1 wherein the anion of said second aqueous solution is selected from the group consisting of molybdate, acid phosphate, silicate, chromate and dichromate. 